250 J. E. SPURR ORIGIN AND STRUCTURE OF THE BASIN RANGES 



Was it unusual rapidity of warping or abnormal inactivit}^ of erosion 

 which brought about the reversal of their ordinary relation ? Other 

 regions with more legible records aid in deciding. The great faults of the 

 Colorado plateau seem to be among the most striking examples of rapid 

 recent deformation. Some have displaced Pleistocene volcanic cones, 

 and their courses are marked at the surface by scarps which are barely 

 defaced by general erosion, yet the Colorado river and its tributaries 

 cross them without any alteration of grade, showing that downcutting 

 has easil}^ kept ahead of differential uplift. No lake would be able to 

 accumulate in the region traversed by these streams unless warping 

 were much more rapid than the faulting has been. 



Viewing examples like this, it seems improbable that the broad Tertiary 

 lake basins under consideration could have long remained undrained 

 had there been even moderately abundant precipitation. Indeed, part 

 of the dejoosits of the Upper Eocene lake in southeastern California are 

 of such a character as to indicate that a great inland sea underwent ex- 

 tensive evaporation, resulting in chemical precipitation from its waters. 

 Therefore the general period of aridity had already begun at this time. 



It is certain, however, that there were important fluctuations. In the 

 same upper Eocene lake series tliat contain borax, gypsum, and calca- 

 reous tufa are beds with coal, fossil leaves, and silicified forests, testify- 

 ing to moist intervals between the arid epoclis. From the little we know 

 of the Great Basin Miocene, it is })robable tliat the same alterations of 

 moist and dr}' climate took })lace. Most of the Pliocene must have been 

 extremely dry, for the great Shoshone lake, marking a period of increased 

 |)recipitation, aj)parent]y did not originate till near the close of the period, 

 if we judge from its fossils, its relation to Pleistocene lakes, and the com- 

 parative freshness of its topographic work ; and among these very lake- 

 beds are some which are impregnated with alkaline carbonates, as if they 

 had been deposited in saline waters.'*^ Major Dutton f considered that the 

 -Pliocene in the Grand Canyon region was a time of aridity. 



Within the Pleistocene the oscillations of climate are well known. 

 King found in the deposits of lake Lahontan evidence of four alternat- 

 ing episodes of moisture and aridity, beginning Avith a moist period and 

 ending with the present dr}^ one. Subsequently Professor Russell X V^^- 

 (ixed to this series the dry })re-Lahontan episode, making five alterna- 

 tions. The succession is the same as that discerned by Mr Gilbert § in 

 the history of lake Bonneville. 



*King : Explonitions of the Fortieth Pivralli;!, vol. i, p. 439, 

 f Second Ann. Kept. U. S. Geol. Survey, p. 120. 

 I Monograph xi, U. S. Geol. Survey, p, 201. 

 g Monograph i, U. S. Geol. Survey, p. 316, 



